1. Field of the Invention
Certain aspects of the present invention relate generally to precise timing measurements for integrated circuits. Other aspects relate to the measurement of phase shifts in high frequency phase modulators.
2. Description of Background Information
Phase modulators are devices that modify the phase of an input signal, and may be used, for example, to synchronize the phase of an input signal to that of a local oscillator. In computer hard disk drives, phase modulators align data being read from the hard disk media to a sampling clock. When writing data to the hard disk media, phase modulators are often used to shift the phase of bit patterns on their way to the disk media.
Precision and linearity are factors frequently considered in phase modulator design. Precision, as used herein, refers to the ability of the phase modulator to repeatedly reproduce the desired amount of shift. Linearity refers to the ability of the phase modulator to produce a phase shift that is proportional to a control signal. Precision and linearity are particularly important in high performance phase modulators. For example, to test the precision and linearity of phase modulators used in modern high performance disk drives, a resolution of 10 picoseconds or less is desirable.
Conventional techniques for measuring the performance of phase modulators use a simple time interval measurement scheme in which a phase shifted version of a reference clock signal is compared to the original version of the reference clock signal. FIG. 1 is a diagram illustrating such a conventional system.
Integrated circuit 100 includes a phase modulator 102, a frequency synthesizer 104, and logic and driver circuitry 106. Frequency synthesizer 104 generates a range of possible frequencies based on a reference clock signal 108. By varying the frequency of the signal output from frequency synthesizer 104, frequency synthesizer 104 may test phase modulator 102 over a range of frequencies Logic and driver circuitry 106 receives the phase shifted signal from phase modulator 102 and transmits it off integrated circuit 100, as output signal 107, to an external testing system (not shown). The external testing system compares output signal 107 with reference signal 108 to ensure that the phase difference between signals 106 and 107 is within acceptable tolerances of the expected phase shift from phase modulator 102. Typically, this comparison is made over a range of frequencies and over a range of phase shift amounts.
The above-discussed technique for testing a phase modulator becomes increasing less useful as the frequency of the signal being phase shifted increases. For example, at frequencies in the order of 100 MHz and above, testing resolutions of 10 picoseconds or less are desirable. Such small resolution times, however, are not easily resolvable with external testing circuitry that directly compares the output signals. In particular, test precision is degraded by phase noise introduced at frequency synthesizer 104, at logic and driver circuitry 106, and in transmitting the high frequency signals through cables to the external test circuitry. Additionally, the act of simultaneously measuring and comparing high frequency signals 107 and 108 requires complicated circuitry that may itself introduce phase noise.
Accordingly, there is a need in the art to be able to more accurately test phase modulators (e.g., chip mounted phase modulators) operating at high frequencies.